Can you use 5-30 oil instead of sae30 in your cultivator?

It is not recommended. Multi blend oils are intended for larger engines to keep lubricant flowing while the engine warms up. In a car engine the oil will heat up at approximately the same rate that the engine warms so the 5-30 is gradually becoming 30. In a small engine the engine will heat up much faster than the oil so you will have a very thin 5 blend trying to lubricate an already warm engine.